The present invention relates to a light emitting apparatus, and specifically to a light emitting apparatus employing a surface emitting laser (SEL) with improved light emitting characteristics.
In general, a semiconductor laser including an edge emitting laser and a SEL is widely used as a small light emitting apparatus. The amount of light emitted from the semiconductor laser must be maintained at a constant level and a monitoring photodetector is used for this purpose.
Since the SEL emits light in the direction of stacked semiconductor layers, several SEL's can be integrated onto a single substrate. In addition, because the shape of the SEL beam is nearly circular and the light intensity shows a Gaussian distribution, there is no need to provide an additional optical device to correct the shape of the emitted light. Therefore, the SEL is widely used as a light emitting apparatus in various technical fields, such as computers, audio/video devices, laser printers, laser scanners, medical devices, and communication systems.
However, since one of the light emitting surfaces of the SEL is combined with the top face of a substrate, installing a monitoring photodetector is not easy.
Accordingly, research into making a photodetector and a SEL of a single body has been actively conducted.
As illustrated in FIG. 1, a conventional light emitting apparatus is composed of a SEL 20 that emits light in the direction of the stacked layers, and a monitoring photodetector 50 disposed on top of the SEL 20, in which the photodetector and the SEL are formed of a single body.
The SEL 20 is comprised of a substrate 1, an active layer 23 which is formed on the substrate 1 and emits light, first and second reflection layers 22 and 27, which are made by stacking semiconductor compounds alternately on the top and the bottom faces of the active layer 23 and which resonate the light emitted from the active layer 23, a high resistance portion 25 formed by injecting ions or protons into the region except for the center portion 23a of the active layer 23, a window 29 for the transmission of light on top of the monitoring photodetector 50, and first and second electrodes 10 and 30 formed on the bottom surface of the substrate 1 and the top surface of the second reflection layer 27, respectively.
When the first and the second electrodes 10 and 30 are biased in the positive direction, an electric current flows through the center portion 23a of the active layer 23 by the high resistance portion 25 and light is produced by combination of electrons and positive holes. Only the light of a wavelength sufficient for resonance condition of the first and the second reflection layers 22 and 27 remains among the produced light, and the light is amplified by the stimulated emission of light having the same phase and wavelength in the active layer 23 by the remaining light. The laser beam produced by the stimulated emission emits light towards the top and bottom surfaces of the SEL 20.
The SEL 20 emits not only the stimulated emitted light but also the spontaneously emitted light, which is a mixed light of several wavelengths and different phases and has a characteristic different from that of the stimulated emission light.
The monitoring photodetector 50 includes a pair of different type impurity semiconductor material layers 51 and 58, an intrinsic semiconductor material layer 57 disposed between these impurity semiconductor layers 51 and 58, and a third electrode 70 for outputting signals detected in the monitoring photodetector 50. The monitoring photodetector absorbs some of the light emitted from the top surface of the SEL 20 and outputs electrical signals in proportion to the absorbed quantity of light.
In the operation of the conventional light emitting apparatus having this structure, some of the light emitted from the top surface of the SEL 20 is absorbed in the monitoring photodetector 50 and the rest of the light is transmitted and emitted through the window 29. Here, the emission of light in the light emitting apparatus is controlled to be constant by detecting the emitted quantity of light from the SEL 20 by means of the received quantity of light in the monitoring photodetector 50.
However, the light detected in the monitoring photodetector 50 includes some of the spontaneously emitted light as well as the stimulated emitted light. Thus, as the detection signals of this spontaneously emitted light have an effect on the detection signals of the monitoring photodetector 50, precise control of the amount of light emitted from the SEL 20 is difficult.